Designing an effective HIV vaccine is a many-faceted challenge. The vaccine preferably elicits an immune response capable of either preventing infection or, minimally, controlling viral replication if infection occurs, despite the failure of immune responses to natural infection to eliminate the virus (Nabel, Vaccine 20:1945-1947 (2002)) or to protect from superinfection (Altfeld et al, Nature 420:434-439 (2002)). Potent vaccines are needed, with optimized vectors, immunization protocols, and adjuvants (Nabel, Vaccine 20:1945-1947 (2002)), combined with antigens that can stimulate cross-reactive responses against the diverse spectrum of circulating viruses (Gaschen et al, Science 296:2354-2360 (2002), Korber et al, Br. Med. Bull. 58:19-42 (2001)). The problems that influenza vaccinologists have confronted for decades highlight the challenge posed by HIV-1: human influenza strains undergoing antigenic drift diverge from one another by around 1-2% per year, yet vaccine antigens often fail to elicit cross-reactive B-cell responses from one year to the next, requiring that contemporary strains be continuously monitored and vaccines be updated every few years (Korber et al, Br. Med. Bull. 58:19-42 (2001)). In contrast, co-circulating individual HIV-1 strains can differ from one another by 20% or more in relatively conserved proteins, and up to 35% in the Envelope protein (Gaschen et al, Science 296:2354-2360 (2002), Korber et al, Br. Med. Bull. 58:19-42 (2001)).
Different degrees of viral diversity in regional HIV-1 epidemics provide a potentially useful hierarchy for vaccine design strategies. Some geographic regions recapitulate global diversity, with a majority of known HIV-1 subtypes, or clades, co-circulating (e.g., the Democratic Republic of the Congo (Mokili & Korber, J. Neurovirol 11(Suppl. 1):66-75 (2005)); others are dominated by two subtypes and their recombinants (e.g., Uganda (Barugahare et al, J. Virol. 79:4132-4139 (2005)), and others by a single subtype (e.g., South Africa (Williamson et al, AIDS Res. Hum. Retroviruses 19:133-144 (2003)). Even areas with predominantly single-subtype epidemics must address extensive within-clade diversity (Williamson et al, AIDS Res. Hum. Retroviruses 19:133-44 (2003)) but, since international travel can be expected to further blur geographic distinctions, all nations would benefit from a global vaccine.
Presented herein is the design of polyvalent vaccine antigen sets focusing on T lymphocyte responses, optimized for either the common B and C subtypes, or all HIV-1 variants in global circulation [the HIV-1 Main (M) group]. Cytotoxic T-lymphocytes (CTL) directly kill infected, virus-producing host cells, recognizing them via viral protein fragments (epitopes) presented on infected cell surfaces by human leukocyte antigen (HLA) molecules. Helper T-cell responses control varied aspects of the immune response through the release of cytokines. Both are likely to be crucial for an HIV-1 vaccine: CTL responses have been implicated in slowing disease progression (Oxenius et al, J. Infect. Dis. 189:1199-208 (2004)); vaccine-elicited cellular immune responses in nonhuman primates help control pathogenic SIV or SHIV, reducing the likelihood of disease after challenge (Barouch et al, Science 290:486-92 (2000)); and experimental depletion of CD8+ T-cells results in increased viremia in SIV infected rhesus macaques Schmitz et al, Science 283:857-60 (1999)). Furthermore, CTL escape mutations are associated with disease progression (Barouch et al, J. Virol. 77:7367-75 (2003)), thus vaccine-stimulated memory responses that block potential escape routes may be valuable.
The highly variable Env protein is the primary target for neutralizing antibodies against HIV; since immune protection will likely require both B-cell and T-cell responses (Moore and Burton, Nat. Med. 10:769-71 (2004)), Env vaccine antigens will also need to be optimized separately to elicit antibody responses. T-cell-directed vaccine components, in contrast, can target the more conserved proteins, but even the most conserved HIV-1 proteins are diverse enough that variation is an issue. Artificial central-sequence vaccine approaches (e.g., consensus sequences, in which every amino acid is found in a plurality of sequences, or maximum likelihood reconstructions of ancestral sequences (Gaschen et al, Science 296:2354-60 (2002), Gao et al, J. Virol. 79:1154-63 (2005), Doria-Rose et al, J. Virol. 79:11214-24 (2005), Weaver et al, J. Virol., in press)) are promising; nevertheless, even centralized strains provide limited coverage of HIV-1 variants, and consensus-based reagents fail to detect many autologous T-cell responses (Altfeld et al, J. Virol. 77:7330-40 (2003)).
Single amino acid changes can allow an epitope to escape T-cell surveillance; since many T-cell epitopes differ between HIV-1 strains at one or more positions, potential responses to any single vaccine antigen are limited. Whether a particular mutation results in escape depends upon the specific epitope/T-cell combination, although some changes broadly affect between-subtype cross-reactivity (Norris et al, AIDS Res. Hum. Retroviruses 20:315-25 (2004)). Including multiple variants in a polyvalent vaccine could enable responses to a broader range of circulating variants, and could also prime the immune system against common escape mutants (Jones et al, J. Exp. Med. 200:1243-56 (2004)). Escape from one T-cell receptor may create a variant that is susceptible to another (Allen et al, J. Virol. 79:12952-60 (2005), Feeney et al, J. Immunol. 174:7524-30 (2005)), so stimulating polyclonal responses to epitope variants may be beneficial (Killian et al, Aids 19:887-96 (2005)). Escape mutations that inhibit processing (Milicic et al, J. Immunol. 175:4618-26 (2005)) or HLA binding (Ammaranond et al, AIDS Res. Hum. Retroviruses 21:395-7 (2005)) cannot be directly countered by a T-cell with a different specificity, but responses to overlapping epitopes may block even some of these escape routes.
The present invention relates to a polyvalent vaccine comprising several “mosaic” proteins (or genes encoding these proteins). The candidate vaccine antigens can be cocktails of k composite proteins (k being the number of sequence variants in the cocktail), optimized to include the maximum number of potential T-cell epitopes in an input set of viral proteins. The mosaics are generated from natural sequences: they resemble natural proteins and include the most common forms of potential epitopes. Since CD8+ epitopes are contiguous and typically nine amino-acids long, sets of mosaics can be scored by “coverage” of nonamers (9-mers) in the natural sequences (fragments of similar lengths are also well represented). 9-Mers not found at least three times can be excluded. This strategy provides the level of diversity coverage achieved by a massively polyvalent multiple-peptide vaccine but with important advantages: it allows vaccine delivery as intact proteins or genes, excludes low-frequency or unnatural epitopes that are not relevant to circulating strains, and its intact protein antigens are more likely to be processed as in a natural infection.